HCV is a positive strand RNA virus belonging to the Flaviviridae family. It is the major cause of non-A non-B viral hepatitis. HCV has infected approximately 200 million people and current estimates suggest that as many as 3 million individuals are newly infected each year. Approximately 80% of those infected fail to clear the virus; a chronic infection ensues, frequently leading to severe chronic liver disease, cirrhosis and hepatocellular carcinoma. Current treatments for chronic infection are ineffective and there is a pressing need to develop preventative and therapeutic vaccines.
Due to the error-prone nature of the RNA-dependent RNA polymerase and the high replicative rate in vivo, HCV exhibits a high degree of genetic variability. HCV can be classified into six genetically distinct genotypes and further subdivided into at least 70 subtypes, which differ by approximately 30% and 15% at the nucleotide level, respectively. A significant challenge for the development of vaccines will be identifying protective epitopes that are conserved in the majority of viral genotypes and subtypes. This problem is compounded by the fact that the envelope proteins, the natural target for the neutralizing response, are two of the most variable proteins.
The envelope proteins, E1 and E2, are responsible for cell binding and entry. They are N-linked glycosylated transmembrane proteins with an N-terminal ectodomain and a C-terminal hydrophobic membrane anchor. In vitro expression experiments have shown that E1 and E2 proteins form a non-covalent heterodimer, which is proposed to be the functional complex on the virus surface. Due to the lack of an efficient culture system, the exact mechanism of viral entry is unknown. That said, there is mounting evidence that entry into isolated primary liver cells and cell lines requires interaction with the cell surface receptors CD81 and Scavenger Receptor Class B Type 1 (SR-B1), although these receptors alone are not sufficient to allow viral entry.
Current evidence suggests that cell mediated immunity is pivotal in clearance and control of viral replication in acute infection. However, surrogate models of infection, such as animal infection and cell and receptor binding assays, have highlighted the potential role of antibodies in both acute and chronic infection. Unsurprisingly, neutralizing antibodies recognize both linear and conformational epitopes. The majority of antibodies that demonstrate broad neutralization capacity are directed against conformational epitopes within E2. Induction of antibodies recognizing conserved conformational epitopes is extremely relevant to vaccine design, but this is likely to prove difficult, as the variable regions appear to be immuno-dominant. One such immuno-dominant linear epitope lies within the first hypervariable region of E2 (HVR1). The use of conserved HVR1 mimotopes has been proposed to overcome problems of restricted specificity, but it is not yet known whether this approach will be successful.
A region immediately downstream of HVR1 contains a number of epitopes. One epitope, encompassing residues 412-423 and defined by the monoclonal antibody AP33, inhibits the interaction between CD81 and a range of presentations of E2, including soluble E2, E1E2 and virus-like particles. See Owsianka A. et al., J Gen Virol 82:1877-83 (2001).
WO 2006/100449 teaches that the monoclonal antibody designated AP33 can bind to and neutralize each of the six known genotypes 1-6 of HCV. Accordingly, it is deduced that the epitope targeted by AP33 is cross-reactive with all of genotypes 1-6 of HCV, indicating it as a target for anti-HCV ligands and as an immunogen for raising anti-HCV antibodies.
AP33 is a mouse antibody, and as such is likely to raise a human anti-mouse antigenic response (HAMA) in human patients if used therapeutically for multiple administrations. There is accordingly a need for an antibody which shares the cross-reactivity of AP33, but which possesses reduced antigenicity in human subjects.